Mammoth in ship's jail right to escape (3)
I believe the answer is:
big
'mammoth' is the definition.
(similar in meaning)
'ship's jail right to escape' is the wordplay.
'ship's jail' becomes 'brig' (I've seen this in another clue).
'right' becomes 'r' (common abbreviation).
'to escape' suggests deleting specific letters.
'brig' with 'r' removed is 'BIG'.
'in' is the link.
(Other definitions for big that I've seen before include "Huge" , "Important" , "Sizeable" , "London bell" , "Of considerable size" .)